The daily light cycle directly and acutely affects many physiological functions in the vertebrate eye resulting in so called "diurnal rhythms". However, it is not simply light or dark adaptation that allows for the daily rhythms in photoreceptor function. A number of retinal processes, from gene expression in photoreceptors to modulation of visual sensitivity, continue to rhythmically oscillate with a period of about 24-hours in the absence of any environmental fluctuations. What accounts for these circadian rhythms that persist in the absences of environmental timing cues? Circadian rhythms are generated by a physiological timing mechanism with the properties of a self-sustaining oscillator. Data clearly indicated that a circadian oscillator is present in some photoreceptors, however, virtual nothing is known about the molecular and biochemical components of the oscillator or the cellular mechanisms by which it regulates visual physiology. What is certain is that understanding the oscillator has very important and fundamental implications for studies of photoreceptors physiology. Results indicate that the circadian system plays a significant role in the regulation of normal photoreceptor structure and function, and disruptions in retinal circadian rhythmicity have been implicated in both inherited and light- induced retinal degeneration. The circadian oscillator which regulates gene transcription of the cone photopigment, iodopsin, is expressed in a dispersed retinal cell preparation of embryonic chick or quail retina. This observation is significant because it provides an in vitro preparation in which to study the ocular clock using biochemical and molecular probes. The specific goals of this proposal are 1) to understand how the iodopsin gene is regulated by light and the circadian clock, and to determine whether all photoreceptors or only a subset are circadian oscillators, 2) to determine when to clock develops in the eye and if it effects photoreceptor differentiation, 3) to develop new procedures which will allow us to study gene regulation in specific photoreceptor subtypes to identify novel genes involved in the generation of circadian oscillations. The experiments posed in this proposal will increase our understanding of how the cellular oscillator is organized and affects photoreceptor physiology. By understanding how the circadian oscillator regulates gene expression, we should begin to understand the biological basis of circadian rhythms that ultimately should lead to procedures useful in the diagnosis and treatment of pathological conditions in retina.